tag added
From: "David Duffy (AVD)" <spam_OUTdavidTakeThisOuTaudiovisualdevices.com.au>
> Hi guys,
> I have a small board that I'd like to drive a 5V reed relay straight
> from a port pin of a small PIC. (cost and size is an issue) The coil is
> 500 Ohms so only draws10mA @5V. Are there any gotcha's with doing this?
> I was going to use a 1N914 as the back EMF diode.

We have a design where an 18F is directly driving a reed relay, with a
1N4148 across the coil. Devices have been in use for two years.

I'm curious, what was your reasoning for going with a reed relay, as opposed
to a solid state device?

> tag added
> From: "David Duffy (AVD)" <.....davidKILLspam@spam@audiovisualdevices.com.au>
>> Hi guys,
>> I have a small board that I'd like to drive a 5V reed relay straight
>> from a port pin of a small PIC. (cost and size is an issue) The coil is
>> 500 Ohms so only draws10mA @5V. Are there any gotcha's with doing this?
>> I was going to use a 1N914 as the back EMF diode.
>
> We have a design where an 18F is directly driving a reed relay, with a
> 1N4148 across the coil. Devices have been in use for two years.
>
> I'm curious, what was your reasoning for going with a reed relay, as
> opposed
> to a solid state device?
>
> Vitaliy

Some reed relays even include the clamp diode inside the relay! A reed is
good where you need to do floating dry switching. Doing that solid state
can be expensive...

"Harold Hallikainen" wrote:
>>> I have a small board that I'd like to drive a 5V reed relay straight
>>> from a port pin of a small PIC. (cost and size is an issue) The coil is
>>> 500 Ohms so only draws10mA @5V. Are there any gotcha's with doing this?
>>> I was going to use a 1N914 as the back EMF diode.
>>
>> We have a design where an 18F is directly driving a reed relay, with a
>> 1N4148 across the coil. Devices have been in use for two years.
>>
>> I'm curious, what was your reasoning for going with a reed relay, as
>> opposed
>> to a solid state device?
>>
>> Vitaliy
>
>
> Some reed relays even include the clamp diode inside the relay! A reed is
> good where you need to do floating dry switching. Doing that solid state
> can be expensive...

"Harold Hallikainen" wrote:
>>> I have a small board that I'd like to drive a 5V reed relay straight

>>> from a port pin of a small PIC. (cost and size is an issue) The coil

>>> is 500 Ohms so only draws10mA @5V. Are there any gotcha's with doing
this?{Quote hidden}

>>> I was going to use a 1N914 as the back EMF diode.
>>
>> We have a design where an 18F is directly driving a reed relay, with
>> a
>> 1N4148 across the coil. Devices have been in use for two years.
>>
>> I'm curious, what was your reasoning for going with a reed relay, as
>> opposed to a solid state device?
>>
>> Vitaliy
>
>
> Some reed relays even include the clamp diode inside the relay! A reed

> is good where you need to do floating dry switching. Doing that solid
> state can be expensive...

> A Health and Safety notice came around work the other day. Anyone
> familiar with UK mains power plugs will know that each one has a fuse
> in it, the rating of which should suit the appliance and its attached cable
> rating. The photo of the plug on the poster showed an M4 bolt fitted in
> the plug ...

Hehe. What's on the other end of that - a rail gun ?

Many's the guitar amp I've seen with cooking foil or that from a pack of
cigarettes jammed into the fuse holder

"Wet" means that there is (DC) voltage present. I think that it used
to mean the DC supply and load of a telephone handset (48~54Vdc,
20mA~100mA) but is now generally referred to as pretty much any
supply and load.

"Dry" means that the (relay or switch) contacts are isolated and not
connected to any power supply inside the device where the relay is located.

This may seem not obvious, but any contact that is referred to as
"Dry" is usually a low-current-capable contact and not normally
suitable for controlling heavy loads directly. Do note that I said
"usually". Low-current-capable means that the contact is gold-plated
or otherwise capable of handing low current signals (does not require
wetting current).

> At 04:22 PM 1/12/2009, Vitaliy wrote:
>
>>PAUL James wrote:
>> >> What's "floating dry switching"? :)
>> > Switching that isn't referenced to ground.
>>
>>Why is it "dry"?
>
> "Wet" and "Dry" are old telephone-industry terms.
>
> "Wet" means that there is (DC) voltage present. I think that it used
> to mean the DC supply and load of a telephone handset (48~54Vdc,
> 20mA~100mA) but is now generally referred to as pretty much any
> supply and load.
>
> "Dry" means that the (relay or switch) contacts are isolated and not
> connected to any power supply inside the device where the relay is
> located.
>
> This may seem not obvious, but any contact that is referred to as
> "Dry" is usually a low-current-capable contact and not normally
> suitable for controlling heavy loads directly. Do note that I said
> "usually". Low-current-capable means that the contact is gold-plated
> or otherwise capable of handing low current signals (does not require
> wetting current).
>
> dwayne

I think that pretty much summarizes it. If you try to switch low current
loads with relays that are designed for power loads, the results will not
be reliable. I learned this years ago when I made a "Yodeling Clock" for
the Madonna Inn in San Luis Obispo, CA. There were two volume controls,
one for day, one for night. A clock drove a relay that switched between
the two. The relay I used was rated for something like 1A at 120VAC. Half
the time there was no audio. A relay rated for dry switching would have
handled it fine. These often are "bifrucated gold cross point" contacts.
There are two contacts in parallel, and each is a gold wedge that make
contact with another gold wedge at 90 degrees so the contact pressure is
high.

Back on wet and dry, telephone cable splices are quite often unreliable
unless a DC "sealing current" is run through them.

>
> These often are "bifrucated gold cross point" contacts.
> There are two contacts in parallel, and each is a gold wedge that make
> contact with another gold wedge at 90 degrees so the contact pressure is
> high.
>
>
Is there the same distinction for reed relays? (different reed relays
for signal use than for power).

Just curious (and because I have a background project to identify information
transfer issues in engineering education) - how many folks were taught this
(below) in their degree program, either in the lab (taught in the lab), or
formal class, or in theory resulting in this bit of knowledge? Also, did anyone
stumble across it during labs in trying to get something to work and got
'experience' this way instead (not formally taught but needed the knowledge to
complete the course)?

>
>
> I think that pretty much summarizes it. If you try to switch low current
> loads with relays that are designed for power loads, the results will not
> be reliable. I learned this years ago when I made a "Yodeling Clock" for
> the Madonna Inn in San Luis Obispo, CA. There were two volume controls,
> one for day, one for night. A clock drove a relay that switched between
> the two. The relay I used was rated for something like 1A at 120VAC. Half
> the time there was no audio. A relay rated for dry switching would have
> handled it fine. These often are "bifrucated gold cross point" contacts.
> There are two contacts in parallel, and each is a gold wedge that make
> contact with another gold wedge at 90 degrees so the contact pressure is
> high.
>
> Back on wet and dry, telephone cable splices are quite often unreliable
> unless a DC "sealing current" is run through them.
>
> Harold
>

I guess that I learned it both the hard way (school of hard knocks)
as well as having friends in the Telecom business who took the time
to explain the how's and why's of wetting current and sealing
current. I was a full-time broadcast engineer at that time (mid
'70s) - wondering why some broadcast loops had high levels of what
seemed to be crossover distortion. Pass some tens of mA through the
loop - distortion was gone. Bad splices somewhere. That would also
be about the time I learned the differences between various relay
types - why some were good for switching audio and some were not.

Certainly nothing about this in school.

I still (occasionally) get bit by wetting current problems - the most
recent was about 18 months ago. That was designing a new controller
around an existing switch assembly.

>Just curious (and because I have a background project to identify information
>transfer issues in engineering education) - how many folks were taught this
>(below) in their degree program, either in the lab (taught in the lab), or
>formal class, or in theory resulting in this bit of knowledge? Also,
>did anyone
>stumble across it during labs in trying to get something to work and got
>'experience' this way instead (not formally taught but needed the
>knowledge to
>complete the course)?
>
>-Skip
>
>Harold Hallikainen wrote:
> >
> > I think that pretty much summarizes it. If you try to switch low current
> > loads with relays that are designed for power loads, the results will not
> > be reliable. I learned this years ago when I made a "Yodeling Clock" for
> > the Madonna Inn in San Luis Obispo, CA. There were two volume controls,
> > one for day, one for night. A clock drove a relay that switched between
> > the two. The relay I used was rated for something like 1A at 120VAC. Half
> > the time there was no audio. A relay rated for dry switching would have
> > handled it fine. These often are "bifrucated gold cross point" contacts.
> > There are two contacts in parallel, and each is a gold wedge that make
> > contact with another gold wedge at 90 degrees so the contact pressure is
> > high.
> >
> > Back on wet and dry, telephone cable splices are quite often unreliable
> > unless a DC "sealing current" is run through them.
> >
> > Harold

> I guess that I learned it both the hard way (school of hard knocks)
> as well as having friends in the Telecom business who took the time
> to explain the how's and why's of wetting current and sealing
> current. I was a full-time broadcast engineer at that time (mid
> '70s) - wondering why some broadcast loops had high levels of what
> seemed to be crossover distortion. Pass some tens of mA through the
> loop - distortion was gone. Bad splices somewhere. That would also
> be about the time I learned the differences between various relay
> types - why some were good for switching audio and some were not.
>
> Certainly nothing about this in school.
>
> I still (occasionally) get bit by wetting current problems - the most
> recent was about 18 months ago. That was designing a new controller
> around an existing switch assembly.
>
> dwayne

About the same here. Relays did not exist in school. But, starting in
junior high, I started messing with telephone systems. I built my first
step switch when in 7th grade out of old pinball machine parts. In high
school, I got a bunch of Strowger switches and built another step switch.
The relay contact issue I learned when building the previously mentioned
yodeling clock. At that time (early 1970s), I also worked in broadcast.

Dr Skip wrote:
> Just curious (and because I have a background project to identify
> information transfer issues in engineering education) - how many
> folks were taught this (below) in their degree program, either in the
> lab (taught in the lab), or formal class, or in theory resulting in
> this bit of knowledge? Also, did anyone stumble across it during labs
> in trying to get something to work and got 'experience' this way
> instead (not formally taught but needed the knowledge to complete the
> course)?

I don't remember where I first heard about wet/dry relay contacts, but it
wasn't in school. I think it was probably from a senior engineer at my
first job out of school, which was for Hewlett Packard.

Dr Skip wrote:
> Just curious (and because I have a background project to identify
> information transfer issues in engineering education) - how many
> folks were taught this (below) in their degree program, either in the
> lab (taught in the lab), or formal class, or in theory resulting in
> this bit of knowledge? Also, did anyone stumble across it during labs
> in trying to get something to work and got 'experience' this way
> instead (not formally taught but needed the knowledge to complete the
> course)?

I don't specifically remember learning about it in training, but do seem to
remember coming across it in discussion among work members during my
apprenticeship.